1. Field of the Invention
The present invention relates to an organic electroluminescent (herein after, may be abbreviated as organic EL) element.
2. Description of the Related Art
The organic EL element has an advantage that visibility is high due to self light emission, impact resistance is excellent, different from a liquid crystal display, due to a wholly solid display, a response rate is rapid, scarcely influenced by a temperature change, and a visual angle is large. Utility as a light emitting element in an image displaying apparatus attracts attention.
The organic EL element has a fundamental construction of a laminated structure of anode/light emitting layer/cathode, and a construction in which an anode as a transparent electrode is formed on a substrate such as a glass substrate is usually adopted. In this case, emitted light is taken out from the substrate side (anode side).
On the other hand, in recent years, an attempt has been made to take out emitted light from the cathode side by using a transparent electrode as the cathode, that is, to perform top side light emission. By realizing this top side light emission, when both of the cathode and the anode are transparent electrodes, it enables to form a transparent light emitting element as a whole, and double side light emission can be realized. In such the transparent light emitting element, since an arbitrary color can be adopted as a background color, a colored display, even when light is not emitted, becomes possible, and ornamentality is improved. In addition, when a color filter layer or a color converting layer is formed on a light emitting element, the color filter layer or the color converting layer can be arranged and laminated on the light emitting layer, as in conventional light emitting element of bottom side light emission, in the light emitting element capable of performing top side light emission. Further, in the light emitting element capable of performing top side light emission, light emission is not shielded by TFT (thin film transistor) of an active driving display, and a display having a high opening rate becomes possible.
As an example of an organic EL element in which top side light emission is made possible by adopting a transparent electrode as the cathode, Japanese Patent Application Laid-Open (JP-A) No. 10-162959 discloses an organic EL element in which an organic EL layer containing a light emitting layer lies between an anode and a cathode, the cathode consists of an electron injecting layer and a transparent electrode layer, and this electron injecting layer is arranged to be contact with the organic EL layer.
However, in the conventional organic EL element in which top side light emission or double side light emission is possible, the transparent electrode such as ITO is generally formed into a film by a sputtering method. And there arises a problem that, upon formation of the transparent electrode, the organic EL layer containing the light emitting layer, and the electron injecting layer or the hole injecting layer are shocked by sputtered particles, Ar+ at sputtering, and an ionized electron, so that the light emitting property is deteriorated (reduction in current density, reduction in light emitting efficiency, leakage of current). Further, there is also a problem that, upon formation of the transparent electrode as the cathode, a highly reactive metal contained in the electron injecting layer is oxidized due to oxygen introduction or release of oxygen from a target, and deterioration in property of the organic EL layer and the electron injecting layer (reduction in current density property, reduction in light emitting efficiency, increase in dark spot) is caused. When there are these problems, a high quality image display cannot be obtained.
Therefore, in order to improve deterioration in light emitting property or deterioration in property of the organic EL layer or the electron injecting layer due to impact at formation of the cathode film, an attempt has been made to form a buffer layer or a barrier layer between the cathode and the organic EL layer. For example, JP-A No. 2002-75658 discloses an organic EL element using CuPu as a buffer layer for the purpose of preventing damage of an organic EL layer due to sputtering upon formation of a cathode film. In addition, JP-A No. 10-144957 discloses a method for preventing a short circuit of an organic EL element and deterioration in property by providing a Ca diffusion barrier layer between a cathode and an organic EL layer for preventing diffusion of materials constituting the cathode into the organic EL layer containing the light emitting layer. Further, JP-A No. 10-223377 and JP-A No. 2000-215984 disclose an organic EL element in which a semiconductor such as ZnSe, ZnS and CdS intervenes between a cathode and an organic EL layer to improve electron transport from the cathode. In addition, JP-A No. 10-125469 discloses an organic EL element in which an conductive material layer such as Ag, Mg and TiN intervenes between a cathode and an organic EL layer containing a light emitting layer to reduce a resistance of the cathode.
However, when the aforementioned buffer layer or barrier layer (organic substance, semiconductor etc.) is inserted, in particular, in an organic EL element which is driven under a high current density (organic EL element using a conductive polymer), there is a problem that current density-voltage property is deteriorated as compared with the conventional organic EL element which takes emitted light out from the bottom side.